The current values of the font attributes are held in internal macros.
\f@encoding
\f@family
\f@series
\f@shape
\f@size
\f@baselineskip
\tf@size
\sf@size
\ssf@size
These hold the current values of the encoding, the family, the series, the shape, the size, the baseline skip, the main math size, the `script' math size and the `scriptscript' math size. The last three are accessible only within a formula; outside of math they may contain arbitrary values.
For example, to set the size to 12 without changing the baseline skip:
\fontsize{12}{\f@baselineskip}
However, you should never alter the values of the internal
commands directly; they must only be modified using the low-level
commands like \fontfamily, \fontseries, etc. If you disobey this
warning you might produce code that loops.